Constantine II was born at Arelate, the son of Constantine and Fausta. His date of birth though is reported as being at some time in February AD 317. This date though is of some doubt since it is known that Fausta's son Constantius II was born in August of the same year.
In any case, before the year AD 317 was over, Constantine II was elevated to the rank of Caesar alongside his half brother Crispus. This was part of an agreement between Constantine and Licinius, who simultaneously promoted his own son, Licinius the Younger, to the same position.
In AD 320 and AD 321 Constantine II then held the consulship, first as the colleague of his father, then of Crispus. The fact of Constantine II being made consul, too young even to be able to sign his own name yet, did much to support Licinius' accusation that Constantine was seeking to advance his sons at the expense of Licinius' son. A matter which was a contributing factor in the eventual break between the two Augusti.
But in AD 326 Crispus was executed for (either for treason or adultery). This left Constantine II as the senior Caesar alongside his brother and co-Caesar Constantius II who had been elevated by his father in AD 323.
In AD 332 Constantine II was sent by his father to the Danube to campaign against the Visigoths and their ruler Alaric. Naturally his was a purely ceremonial command, the actually commanding of the troops being conducted by seasoned generals rather than an unexperienced teenage, royal heir.
In AD 335 Constantine announced the division of the empire to follow his own death, between his three sons and his nephews Dalmatius and Hannibalianus. In this division Constantine II would receive Gaul, Spain and Britain.
Though the sons would defy their Constantine's wishes after his death in AD 337. Between them the brothers agreed to simply eliminate their cousins Dalmatius and Hannibalius. Was the reason for the murder of their cousins not have to share territory with them, then Constantine II failed to secure any additional territory for himself, remaining in control of only Britain, Gaul and Spain, though he, the eldest among the brothers, was acknowledged as the senior Augustus by the other two.
Was their very accession to power tainted by murder, then it wasn't long before the brothers began to quarrel among themselves. One particular source of trouble was the bishop Athanasius who after fleeing to Treviri was granted permission by Constantine II to return to Alexandria which was in the domain of Constantius II, who wanted him there under no circumstances.
In an attempt to allay their differences, the brothers held a meeting either somewhere in Pannonia or at Viminacium. Among other things they tried to settle border disputes. But if these negotiations led to Constans gaining additional territory, then Constantine II was once again left only with Britain, Gaul and Spain.
In AD 340 Constantine II broke with Constans and invaded Italy, with Constans absent from Rome engaged in suppressing an uprising among the Danubian tribes.